The design and manufacture of resistance heating elements depends on the wattage required for a given application and the corresponding resistance determined by Ohm""s Law which states that for any circuit the electric current is directly proportional to the voltage and is inversely proportional to the resistance. However, limitations on voltage and current must be observed, subject to the capacity of the power source and safety considerations. For example, a high voltage (e.g., 110, 240 or 480 volts alternating current, VAC) can produce a very high current unless the resistance of the heating element is also very high.
Flexible expanded graphite sheet has a relatively high resistivity along its length and width, and excellent heat conducting and electrical conducting properties that are well suited for use in low voltage heater applications. However, the usefulness of this material for high voltage heater applications has been limited because of the unavailability of flexible expanded graphite heating elements having a sufficiently high electrical resistance.
A variation in the length, width and/or thickness of flexible expanded graphite sheet of a given density can change, by a large magnitude, the electrical resistance and, consequently, the amount of electric current that will flow through the material. For a given length and width, an increase in the thickness of a flexible expanded graphite sheet results in a decrease in the electrical resistance and a higher current flow. For high voltage applications that require high resistance it is therefore desirable to use a flexible expanded graphite heating element that is as thin as possible. However, commercially produced flexible expanded graphite sheet, having densities ranging from 50 to 90 lbs./ft3, is available in thicknesses of 3 to about 60 mils, with a thickness of 15 mils the most common. Flexible expanded graphite sheet having a minimum thickness of 3 mils does not provide a sufficiently high electrical resistance for many high voltage heater applications.
Unexpectedly, it has been discovered that the thickness of a flexible expanded graphite sheet of any density can be substantially decreased to produce a thinner flexible expanded graphite sheet having any desired thickness. Ultra-thin flexible expanded graphite sheets (e.g., having a thickness of about 2 mils or less) produced by the methods of the invention are particularly useful as, but not limited to, electrical resistance heater elements or electrical strip heaters for high voltage heater applications.
In one embodiment of the invention, an ultra-thin flexible expanded graphite heating element is produced by a method comprising the steps of (a) providing a flexible expanded graphite sheet having a surface adhered to a substrate; (b) pulling apart the sheet and the substrate with a force sufficient to separate the adhered flexible expanded graphite sheet into a removed layer and a remainder layer adhered to the substrate; and (c) optionally repeating steps (a) and (b) until the remainder layer has a thickness of about 0.01 mils to about 2 mils.
In another embodiment of the invention, an ultra-thin flexible expanded graphite heating element is produced by a method comprising the steps of (a) providing a flexible expanded graphite sheet having a top surface, and a bottom surface adhered to a first substrate; (b) adhering a second substrate to the top surface; (c) separating the first and second substrates with a force sufficient to separate the flexible expanded graphite sheet into a first remainder layer adhered to the first substrate and a second remainder layer adhered to the second substrate; and (d) optionally repeating steps (a), (b) and (c) until at least one of the remainder layers has a thickness of about 0.01 mils to about 2 mils.
In each of the embodiments described above, the resulting remainder layer of flexible expanded graphite sheet can have a substantially uniform thickness, particularly if the second substrate is uniformly adhered to the top surface. A combination of the above embodiments can also be employed to obtain a remainder layer having a desired thickness.
In another embodiment, a method according to the invention produces a layer on a substrate of flexible expanded graphite sheet that is non-uniform in thickness. A non-uniform thickness of the sheet can be useful in many applications but is especially useful in heater element applications when it is desired to provide a variation in current through different parts of the element. An ultra-thin flexible expanded graphite heating element having a non-uniform thickness is produced by a method comprising the steps of (a) providing a flexible expanded graphite sheet having a top surface, and a bottom surface adhered to a first substrate; (b) non-uniformly adhering a second substrate to the top surface; (c) separating the first and second substrates with a force sufficient to separate the flexible expanded graphite sheet into a first remainder layer adhered to the first substrate and a second remainder layer adhered to the second substrate; and (d) optionally repeating steps (a), (b) and (c) until at least a portion of one of the remainder layers has thickness of about 0.01 mils to about 2 mils.
A combination of this method with the above-described embodiment that does not employ a second substrate can also be used to obtain a sheet that is non-uniform in thickness.
An ultra-thin flexible expanded graphite sheet produced by any of the embodiments of the methods of the invention can have any desired thickness and can retain the density of the original flexible expanded graphite sheet. Optionally, the remainder layer produced by any of the methods can be pressure rolled as a finishing process to recompress the dendritic surface of the graphite to provide surface uniformity. If desired, the remainder layer can be sufficiently compressed by rolling or pressing to produce a thinner and denser graphite. Preferably, the flexible expanded graphite sheet comprises a heating element having a thickness that is suitable for a high voltage heater application. For example, the thickness of the sheet produced by a method according to the invention can be about 0.01 mils to about 2 mils, preferably about 0.01 mils to about 1.5 mils, more preferably about 0.01 mils to about 1 mils, especially about 0.01 mils to about 0.4 mils, and more especially about 0.01 mils to about 0.1 mils, and the like, depending on the heater application for which it is to be used.
Ultra-thin flexible expanded graphite remainder layers produced by any of the embodiments of the invention are adhered to a substrate. The substrate itself can be electrically insulating; however, a non-insulating substrate can be adhered to an insulating substrate prior to use of the flexible expanded graphite as a heating element.
The invention further provides a resistance heater for high voltage applications, comprising an electrically insulating substrate; a flexible expanded graphite sheet having a thickness of about 0.01 mils to about 2 mils; a power source; and a connector for supplying power from the power source to the flexible expanded graphite sheet.